It's funny because I was thinking of posting a prompt for a dark/cruel AU of one of DD's love interests, but had ruled out Claire because despite occasionally helping to torture people, I still can't help seeing Claire as a fluffy bunny.

BUT. After reading your prompt, it made me think in a different direction. I still can't see Claire being sadistic to a loved one, but I can definitely see her being a very, very vengeful sadist to bad guys.

Maybe she snaps after one too many times seeing Matt on the brink of death, hunts down the people who did it to him one by one and uses all her medical expertise to make them suffer.